1. Field of the Invention
This invention relates to a scanner, and more particularly, to a method for aligning a charge coupled device (CCD) of a scanner.
2. Description of the Prior Art
Scanners are commonly used in office environment for scanning document images into computers. A scanner usually comprises a transparent window for allowing light reflected from a document to be scanned back to the scanner, a charge coupled device (CCD) having a plurality of optic sensors for converting a line image reflected from the document into an array of analog signals, a set of lenses for conveying light reflected from the document to the CCD, an analog-to-digital (A/D) converter for converting the analog signal array into an image data array, a control unit for controlling operations of the scanner, and a memory for storing the image data array. If the scanner is a flat bed scanner, the CCD and the lens set are packed inside a housing which is called a scanning module and the whole module is movably mounted on a guiding shaft. A step motor is used to move the scanning module forward and backward along the shaft for scanning a complete document placed above the transparent window of the scanner.
The CCD usually contains more light sensors than what are really needed for scanning a document. For example, it may contain 2750 light sensors which can generate 2750 analog signals or 2750 image data after A/D conversion when converting a line image, but within the 2750 image data only 2550 of them are taken by the scanner as valid image data and the rest of them are usually ignored. The selected 2550 image data within the overall 2750 image data are defined as effective scanning range of the CCD.
The effective scanning range is usually pre-programmed into a scanner by prior art methods. When assembling a new scanner, the physical location of each CCD within the scanner is precisely aligned in the assembly process in order to make sure that the effective scanning range of the CCD can match up with a target area within the transparent window for scanning a document placed on the window. One problem faced by the prior art methods is that even with such a precision alignment step, a scanner may still fail to pass a final effective scanning range test after the scanner is completely assembled.
The reason is that throughout the assembly process mechanical tolerances of various mechanical parts are continually accumulated between the relative position of the CCD and the target area of the transparent window. For example, when assembling a flat bed scanner, after the CCD within a scanning module is precisely aligned, the relative position between the scanning module and the shaft, the location of the transparent window within the scanner housing, the location of the shaft within the scanner housing, the connection part between the upper and lower scanner housing, etc., will all introduce some mechanical variations which may cause a line image reflected a document placed within the target area of the transparent window failed to be completely received by the CCD within the effective scanning range. In this case the scanner must be thoroughly checked and aligned again in order to make sure it can pass the effective scanning range test. In general, the precision alignment process of the CCD is a very time consuming process, and the rework of the scanner for passing the effective scanning range test also consumes a lot of time and effort.